i guess just the no-slip doesn't pop and make funny noises. the powertax is pretty much the same. im just gonna go with an aussie cause from what i have read it will perform the same as either of the powertrax offraod.

Well technically the 8.4 is an 8" axle. Some info on why it is referred to a 8.4":

The 8.25/8.4, whatever is not. It is a 8" gear. The casting is better still. The carrier bearings are shimmed, rather than screw in adjusters. The ring gear has 12 bolts instead of 10. Also, the carrier bearing caps are connected with "girdles" and are cast as one piece. This diff I will simply call the late model v6. The pinion shaft is larger, the bearings are slightly lager and the casting is beefier than earlier versions. However, THE RING GEAR IS NO LARGER THAN EALIER 8" DIFFS!

Well technically the 8.4 is an 8" axle. Some info on why it is referred to a 8.4":

The 8.25/8.4, whatever is not. It is a 8" gear. The casting is better still. The carrier bearings are shimmed, rather than screw in adjusters. The ring gear has 12 bolts instead of 10. Also, the carrier bearing caps are connected with "girdles" and are cast as one piece. This diff I will simply call the late model v6. The pinion shaft is larger, the bearings are slightly lager and the casting is beefier than earlier versions. However, THE RING GEAR IS NO LARGER THAN EALIER 8" DIFFS!

i was curious if an aussie locker was a straight drop in mod or if i would end up adjust my backlash and such. i have the 8.4 rear end with 4.56 gears. non elocker obviously

I think you need to mark and make sure you place the washers, shims and bearing caps on the correct sides or it will be off from original. Whatever came off the right side needs to go back on the right and same with the left.